Generally, a video camera includes a zoom lens for zooming. In a conventional system, the position of the zoom lens is varied by a zoom motor to perform a variable picture enlargement function. To simplify the driving circuit of the zoom motor, the zoom motor is usually driven at a predetermined constant velocity.
Focusing system for the zoom lens of a video camera, including a system wherein an inner focusing system uses a luminance signal and a system wherein a front focusing system uses a infrared beam, are known to provide means for the adjusting focus of a camera.
FIG. 1 is a block diagram useful in explaining the operation and configuration of a video camera employing a conventional inner focusing system. In FIG. 1, optical information regarding the object to be photographed is passed through a zoom lens 1a and a focus lens 1b to a charged coupled device (CCD) 1d, via an iris 1c, which CCD produces an electrical signal. The electrical signal is output as a video signal via a video processor 4.
A position signal of zoom lens 1a (hereinafter referred to as a zoom position signal) detected by a zoom encoder 2 indicates the position of zoom lens 1a. The zoom position signal is digitized by an analog-to-digital (A/D) converter incorporated in a microcomputer 6.
Meanwhile, an automatic focusing (AF) processor 5 detects an AF evaluation value, using the high-band component of a luminance signal of the output signals of video processor 4. A photosensor 3 is commonly employed when an AF motor 8a is a stepping motor to discern the center position of the stepping motor, that is, the center position for focus lens 1b. Photosensor 3 drives AF motor 8a when the system initializes, e.g., when the power is turned on, and locates focus lens 1b on the distance center thereof so that the center of focus lens 1b is sensed to perform the AF function.
Microcomputer 6 outputs a drive signal to an AF motor driving circuit 8 by using the AF evaluation value output from AF processor 5, so as to focus at the maximum AF evaluation point. Furthermore, the microcomputer outputs a drive signal to a zoom motor driving circuit 7 to shift the zoom lens 1a to a zooming position corresponding to a zoom selection signal set by a user.
When zoom motor driving circuit 7 drives a zoom motor 7a using the drive signal, microcomputer 6 decides whether a zoom position has been selected by a user, by a zoom position signal output from zoom encoder 2, to control the zooming operation.
However, in order to keep the viewed size of an object constant with respect to the screen in such a system, i.e., to maintain a constant viewing angle, when the object to be photographed moves forward and backward or a photographer moves forward and backward with respect to the object, the zooming operation is controlled at a fixed zooming velocity using a zoom-in/zoom-out selection key. Since the zooming operation is performed at a predetermined velocity regardless of the velocity at which the object or photographer moves, it is difficult to maintain a constant object size as viewed on the screen. Furthermore, since the zoom-in/zoom-out selection key is manually operated, the skillful use of the zoom lens is rather difficult for novice photographers.
A zoom driving apparatus which does not destroy an in-focus state during the zooming operation is disclosed in U.S. Pat. No. 4,991,944, which is incorporated herein by reference for all purposes. In the apparatus, in order to perform the zooming operation, a photographer must manually operate a zoom switch in an intended direction, and the zooming velocity cannot be varied adaptively with respect to variations of the object within the screen, so that the object cannot be photographed with a fixed size on the screen.